In walk-in refrigerators, freezers, or cold storage rooms the insulation therefor is extremely important. Here the type of insulation is of the block type rather than a fibrous type. Block insulations such as polyurethane, polystyrene, cork, or the like must be securely fastened to rigid walls which in most instances are made of concrete although they can be made of other materials such as wood, steel, or even plastic. The securing of the block insulation pieces to the walls of such a chamber has usually been done by just a cement. Under such a construction continued adherence of the insulation block to the cold chamber walls is questionable. Oftentimes the block falls off and the effectiveness of the cold chamber is immediately lost. Clips and fasteners of many designs have been tried but all found wanting. Such devices usually tore the fragile insulation block to materially reduce its effectiveness and they were not capable of physically holding the block to the rigid wall. Now, with the present invention the parallel rows of thin, elongated spikes which are securely fastened to the rigid wall by a central back plate, easily penetrate into the insulation block for a substantial distance and with the aid of an adhesive coating and inner end undercutting on the spikes the block is fixedly secured to the rigid wall with no opportunity of coming loose and positively sealing the insulation block around the elongated spikes.